There are known electronic and mechanical pipettes with the adjustable volume value of the aspirated liquid, which are built in its upper part of a handle and in its lower part of a nozzle with a conical end for an exchangeable tip as well as of coupled with them and cooperating with each other mechanisms performing different functions in the pipette, namely a pipette exchangeable tip ejection mechanism, a drawing up and discharging mechanism, a plunger working stroke adjustment mechanism, a lock mechanism of the plunger working stroke adjustment mechanism, a drive mechanism of a mechanical counter indicating a liquid volume value that is drawn-up by the pipette, and a calibration mechanism.
The pipette exchangeable tip ejection mechanism comprises an ejector push button seated in the upper part of a pipette handle and an ejector seated at least by its lower end onto a pipette nozzle and, between them, intermediate means for transferring a force from the push button onto the ejector. This mechanism enables an exchange of the pipette tip when it is necessary when the pipette tip is worn or damaged or in the case when the need for the change of the working liquid volume values exists.
The drawing up and discharging mechanism comprises a pipetting button connected via a pipetting rod with a plunger that is placed in the pipette nozzle and is slidable therein in PIPETTING MODE of the pipette from and to its upper position. The plunger is driven towards the exchangeable tip by the force which is manually applied by an user to the pipetting push button. In the opposite direction, the plunger is driven in PIPETTING MODE by the pipetting spring force until it rests on an end stop mounted in the plunger working stroke adjustment mechanism.
In ADJUSTMENT MODE, a position of an upper stop of a plunger working stroke is being changed by means of a drive assembly of an adjustment screw of the plunger working stroke adjustment mechanism. The drive assembly of the plunger working stroke adjustment mechanism has a drive member which is accessible for the user from outside. The plunger working stroke adjustment mechanism controls a length of a plunger movement that is performed during liquid pipetting in PIPETTING MODE.
For ensuring an easy adjustment of the desired drawn-up liquid volume value, in the course of resetting adjustment elements of the plunger working stroke adjustment mechanism in ADJUSTMENT MODE, the torque, which is necessary for the change of the setting, should be as small as possible. Whereas, after the adjustment of the desired volume value during operation with the pipette in PIPETTING MODE, this torque should be sufficiently large to rule out any risk of an incidental pipette resetting to another volume value. This requires the application in the pipette an additional mechanism for changing a braking torque exerted on the plunger working stroke adjustment mechanism or an additional mechanism for disengagement the external adjustment elements of the plunger working stroke adjustment mechanism from the elements changing the position of the stop mounted in the plunger working stroke adjustment mechanism, which position determines the length of the plunger working stroke.
Therefore, for prevention the desired volume value of the drawn-up liquid, which is previously set in ADJUSTMENT MODE, from the accidental resetting during operation with the pipette in PIPETTING MODE, the pipettes are provided with a manually actuated lock mechanism for locking the adjustment mechanism of the plunger working stroke.
In the known pipettes, the lock mechanism of the plunger working stroke adjustment mechanism, which constitutes a releasable friction brake, is more often applied. This lock mechanism provides, after its actuation, the large braking torque exerted on the plunger working stroke adjustment mechanism and preventing in PIPETTING MODE from the unintentional resetting the desired volume value of the liquid previously aspirated by the pipette in ADJUSTMENT MODE. The lock mechanism simultaneously provides, after its release, the small braking torque which is exerted on the plunger working stroke adjustment mechanism and is necessary for adjustment of the desired volume value of the drawn-up liquid.
The plunger working stroke adjustment mechanism is via the drive mechanism of the mechanical counter engaged with the counter for indication the liquid volume value which is drawn-up by the pipette in PIPETTING MODE to the exchangeable tip and which is earlier pre-set by the user in ADJUSTMENT MODE by adjustment of the plunger working stroke. Thus, the length of the plunger movement in the nozzle, that is its working stroke, during pipetting in PIPETTING MODE, is altered by the user due to resetting structure elements of the plunger working stroke adjustment mechanism in ADJUSTMENT MODE. This simultaneously results in the change of the indications of the counter which indicates the pre-set desired liquid volume.
However, in many cases, the actual liquid volume value being drawn-up by the pipette differs from the one which is indicated by the counter. This difference between the actual drawn-up liquid volume value and the liquid volume value indicated by the counter is called as a pipette accuracy deviation. There are many factors affecting these inaccuracies, including, among others, a shape and a wear of the exchangeable tip, a dead volume of the air between the liquid in the exchangeable tip and the plunger, the mutual configuration of the component parts of the engaging mechanism which engages the adjustment mechanism with the counter as well as characteristics of the drawn-up liquid and operational conditions of the pipette. For compensation of the pipette accuracy deviation a calibration of the pipette is being carried out in CALIBRATION MODE.
For calibration, the pipettes are provided with the calibration mechanism which enables the correction of the pipette accuracy deviation in the pipette CALIBRATION MODE.
In the known electronic pipettes, especially with the indication on the counter display of the liquid volume values being drawn-up by the pipette, the pipette calibration is realized by the change of the counter indications into the actual liquid volume values drawn-up by the pipette at the given setting of the plunger working stroke, wherein these actual liquid volume values are determined on the basis of series of the liquid drawn-ups done and their weight measures done and, further, on the basis of the calculated pipette accuracy deviations that is differences between the desired liquid volume values and the actual values drawn-up by the pipette.
While, in the known mechanical pipettes, due to realization difficulties, the correction of the pipette accuracy deviation is possible only when the plunger working stroke adjustment mechanism is disengaged from the mechanical counter. Then, the plunger working stroke setting is being reset without the change of the mechanical counter indications or the mechanical counter indication is being reset without the change of the plunger working stroke setting. For correction of the pipette accuracy deviation, an additional knob or a calibration wrench is often used, resetting of which enables to match the liquid volume values drawn-up by the pipette to the counter indications.
From German patent specification No. DE 43 35 863 C1, there is known the pipette built of the adjustment mechanism for the plunger working stroke adjustment, the mechanical counter for indication of the dispensed liquid volume and the engaging mechanism, placed between the adjustment mechanism and the mechanical counter, for the pipette accuracy deviation correction which is the difference between the actual volume value of the dispensed liquid and the liquid volume value indicated on the counter. The pipette is provided with a change-over device for disengagement of the engaging mechanism and for the correction of the pipette accuracy deviation by resetting of the adjustment mechanism, and precisely, by the change of the position of the upper stop which is mounted in the plunger working stroke adjustment mechanism and onto which the plunger rests in PIPETTING MODE. The engaging mechanism has two, coupled with each other, the clutch face gear wheels, while the adjustment mechanism is coupled with the adjustment face gear wheel that is turned respectively to the plunger stroke, and the mechanical counter is coupled with the indication face gear wheel that is turned respectively to the indicated liquid volume. The first clutch face gear wheel is in gear with the adjustment face gear wheel, while, the second clutch face gear wheel is in gear with the indication face gear wheel. The clutch face gear wheels are arranged in the same axis, are rested on the spring and are movable by the change-over device along the axis in the direction opposite to the direction of the spring action and with the disengagement of at least one face clutch. In this pipette, the plunger working stroke adjustment mechanism is coupled with the mechanical counter via the releasable coupling which is disengaged before resetting the plunger working stroke without the change of the mechanical counter indications. In PIPETTING and ADJUSTMENT MODES, the engaging mechanism between the adjustment mechanism and the mechanical counter is continuously engaged, whereas in CALIBRATION MODE is disengaged and thereby the counter drive is also disengaged.
There is also known the mechanical pipette manufactured by PZ HTL S.A. company under the trademark DISCOVERY, in which the adjustment screw of the plunger working stroke adjustment mechanism is non-releasably connected with the mechanical counter. The adjustment screw while turning in the knob mounted in the pipette handle causes the change of the position of the upper stop being mounted in the adjustment mechanism and cooperating with the plunger of the drawing up and discharging mechanism. The position of the lower stop is constant. Due to such a structure, for calibration of this pipette, when the discrepancy between the counter indication and the actual volume value of the drawn-up liquid is observed, it is necessary to change the upper stop position without the change of the angle position of the adjustment screw. It is possible due to the calibration tap screw that is mounted in the adjustment knob. The change of the angle position of the calibration tap screw causes the change of the upper stop position at the stationary constant angle position of the adjustment screw and thereby the counter indication. Thus, it is possible to harmonise the actual drawn-up liquid volume value with the counter indication.
In the above presented mechanical pipettes, the correct calibration consists in the adaption of the plunger working stroke to the counter indications, which, however, requires the calculation of the angle through which the adjustment elements of the adjustment mechanism should be turned by means of the adjustment knob or the additional calibration tap screw so that the pipette draws up the liquid volume value that is indicated by the counter. These pipettes need to run series of additional evaluations of rotation angles of the respective adjustment elements. It is, thus, necessary during every calibration to use an operation manual in order to calculate through which angle the adjustment knob or the calibration tap screw should be turned for obtaining the desired change in the volume value of the liquid drawn-up by the pipette. This is inconvenient and burdensome for the user. And then, the correctness of the performed calibration should be checked. In case of the large discrepancy between the counter indication and the actual drawn-up liquid volume value, the one-step calibration of the pipette is likely to be impossible and several repetitions of the above listed activities will be necessary for the user to follow again for the correct calibration.
Moreover, there is known the mechanical pipette of BRAND company, which is present on markets under the trade mark Transferpette®S. The calibration in this pipette consists in the adaption of the counter indication to the actual volume value of the liquid drawn-up by the pipette at the given setting of the plunger working stroke and without the change of the setting. The actual volume values of the liquid drawn-up by the pipette at the given setting of the plunger working stroke are determined on the basis of series of the liquid drawn-ups and their weight measures and subsequently evaluated calculations of the pipette accuracy deviations, i.e. the differences between the desired liquid volume values and the actual volume values of the liquid drawn-up by the pipette. For the calibration, in this pipette, it is necessary to disengage, earlier, the plunger working stroke adjustment mechanism from the counter so that the change of the setting of the pipette plunger working stroke is, simultaneously, prevented and thereby the change of the volume value of the liquid drawn-up by the pipette is also prevented.
In this pipette, the change of the position of the upper stop mounted in the adjustment mechanism and cooperating with the plunger of the drawing up and discharging mechanism is realized by turning with the adjustment knob which is coupled, in ADJUSTMENT MODE, via the shape rod with the upper stop. The position of the lower stop is not changed. The adjustment knob is further fixed to the adjustment sleeve which is coupled fixedly via the toothed-wheel rim situated in its lower part with the mechanical counter. In the upper part, the adjustment sleeve is provided with the gear clutch for releasable coupling of the adjustment sleeve with the shape rod coupled with the adjustment knob. The adjustment knob is movable along the pipette axis between the upper extreme position and the lower extreme position. The adjustment knob in its lower extreme position is coupled with the upper stop of the adjustment mechanism and is connected with the adjustment sleeve i.e. is coupled with the counter. The pipette is in ADJUSTMENT MODE. The adjustment knob in its upper extreme position is disengaged from the upper stop but is still coupled with the adjustment sleeve i.e. is coupled with the counter. The pipette is in CALIBRATION MODE. Such a pipette construction enables the change of the mechanical counter indications without the change of the position of the upper stop limiting the axial movement of the plunger in PIPETTING MODE and thereby without the change of the volume value of the liquid drawn-up by the pipette. For changeover of the pipette from ADJUSTMENT MODE to CALIBRATION MODE, the adjustment knob should be moved upwards.
The above described manner of the pipette calibration, in which the pipette calibration is carried out by the counter indications change into the actual volume values of the liquid drawn-up by the pipette at the given plunger working stroke setting, is easier for the user than the one earlier described, in which the plunger working stroke setting is being adapted without the change mechanical counter indications, because it does not require any additional calculations of the angle, by which the calibration knob should be turned for obtaining the correct volume value of the liquid drawn-up. It is enough to change the indication into the actual volume value on the mechanical counter, which is determined at the given plunger working stroke setting on the basis on the number of the liquid drawn-ups and their weight measures.
However, in the described mechanical pipette of the BRAND company, due to the design of the applied coupling between the adjustment knob and the upper stop mounted in the adjustment mechanism, the disadvantageous unintentional resetting of the upper stop position takes place relatively easy, wherein the position of the upper stop determines the plunger working stroke. Thus, the unintentional change of the setting of the liquid volume value during disengagement activity and the subsequent engagement activity of the adjustment knob from and with the upper stop in the course of the changeover of the pipette from PIPETTING MODE or ADJUSTMENT MODE to CALIBRATION MODE and vice versa is quite often observed. If the complete disengagement does not take place, for example, because of the incidental too small upward movement of the adjustment knob or its momentary partial downward movement, or the momentary engagement takes place or in the course of the disengagement activity and then the reengagement activity, the unintentional change of the mutual configuration of the cooperating structure elements happens, which causes the disadvantageous uncontrolled change of the setting of the plunger working stroke i.e. the volume value of the liquid drawn-up, and thereby causes the incorrect calibration of the pipette.